News from Gaia on sigma Ori E: a case study for the wind magnetic braking process

Abstract

Context. σ Ori E, a massive helium B-type star, shows a high surface rotation and a strong surface magnetic field potentially challenging the process of wind magnetic braking. Aims. The Gaia satellite provides an accurate distance to that star and confirms its membership to the σ Ori cluster. We account for these two key pieces of information to investigate whether single star models can reproduce the observed properties of σ Ori E and provide new estimates for its metallicity, mass, and age. Methods. We compute rotating stellar models accounting for wind magnetic braking and magnetic quenching of the mass loss. Two metallicities (Z=0.014, with a helium mass fraction Y=0.273 and Z=0.020 with Y = 0.266), four initial masses between 8 and 9 M , three initial rotations between 250 and 450 km s−1 and three initial surface equatorial magnetic field between 3 and 7 kG are considered. Differential rotation is assumed for the internal rotation in all models. We look for models accounting simultaneously for the observed radius, position in the HR diagram, surface velocity, and braking timescale. Results. We obtain that σ Ori E is a very young star (age less than 1 Myr) with an initial mass around 9 M , a surface equatorial magnetic field around 7 kG and having a metallicity Z (mass fraction of heavy elements) around 0.020. No solution is obtained with the present models for a metallicity Z=0.014. The initial rotation of the models fitting σ Ori E is not much constrained and can be anywhere in the range studied here. Because of its very young age, models predict no observable changes of the surface abundances due to rotational mixing. Conclusions. The simultaneous high surface rotation and high surface magnetic field of σ Ori E may simply be a consequence of its young age. This young age implies that the processes responsible for producing the chemical inhomogeneities that are observed at its surface should be rapid. Thus for explaining the properties of σ Ori E, there is no necessity to invoke a merging event although such a scenario cannot be discarded. Other stars (HR 5907, HR 7355, HR 345439, HD 2347, CPD -50o3509) showing similar properties as σ Ori E (fast rotation and strong surface magnetic field) may also be very young stars, although determination of the braking timescales is needed to confirm such a conclusion.